Okay great, so a patch needs to be applied to the ESXi 5.5 host. The customer tried to applied the patch via VUM but it was marked obsoleted. "Obsoleted" is an Update Manager compliance state. As explained in the Installing and Administering VMware vSphere Update Manager documentation:

This compliance state applies mainly to patches. The target object has a newer version of the patch. For example, if a patch has multiple versions, after you apply the latest version to the host, the earlier versions of the patch are in Obsoleted By Host compliance state.

After some discussion we landed on the fact that the vCenter instance managing this host was likely the culprit as it has a patch level several versions behind the host. I always recommend customers keep their vCenter and ESXi hosts at the same Update patch level. Installing patches on hosts in-between major Updates is okay. In this case, they were on different Update versions.

The vCenter instance was patched to the latest update and voila, the error disappeared from vROps.